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Message-ID: <63e10688-5207-e8a4-dac2-d6c6094fbcc2@rasmusvillemoes.dk>
Date: Thu, 1 Nov 2018 23:06:23 +0100
From: Rasmus Villemoes <linux@...musvillemoes.dk>
To: Kees Cook <keescook@...omium.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
"open list:NFS, SUNRPC, AND..." <linux-nfs@...r.kernel.org>,
Trond Myklebust <trond.myklebust@...merspace.com>,
linux-hwmon@...r.kernel.org,
Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
"Steven Rostedt (VMware)" <rostedt@...dmis.org>,
Jean Delvare <jdelvare@...e.com>,
Guenter Roeck <linux@...ck-us.net>
Subject: Re: [RFC PATCH 0/7] runtime format string checking
On 2018-10-30 21:58, Kees Cook wrote:
> On Sat, Oct 27, 2018 at 12:24 AM, Rasmus Villemoes
> <linux@...musvillemoes.dk> wrote:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=ce9b938574042d09920650cb3c63ec29658edc87
> The above seemed to "noisy" to send, but perhaps we should just land
> it anyway. They really _should_ be const.
>
Isn't that 063246641d4a9e9de84a2466fbad50112faf88dc in mainline ;) ?
BTW, I don't agree with all the changes in there: For auto variables, this
- const char *cur_drv, *drv = "acpi-cpufreq";
+ const char drv[] = "acpi-cpufreq";
+ const char *cur_drv;
makes gcc actually generate that string on the stack instead of just
referring to an anonymous object in .rodata; one gets code gen like
+: 31 c0 xor %eax,%eax
+: 48 b8 61 63 70 69 2d movabs $0x7570632d69706361,%rax # "acpi-cpu"
+: 63 70 75
+: c7 44 24 0b 66 72 65 movl $0x71657266,0xb(%rsp) # "freq"
+: 71
+: c6 44 24 0f 00 movb $0x0,0xf(%rsp) "\0"
+: 48 89 44 24 03 mov %rax,0x3(%rsp)
It's not the-end-of-the-world-horrible, but it's better avoided,
especially for patches that are not supposed to change anything. And
longer strings would of course produce even more gunk like the above.
A better fix which also silences -Wformat-security is to declare the
variable itself const, i.e.
const char *const drv = "acpi-cpufreq".
Yes, gcc should be able to infer the constness of drv from the fact that
it's never assigned to elsewhere in the function... I think I saw that
on some gcc todo list at some point.
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=kspp/format-security&id=b7dcfc8f48caaafcc423e5793f7ef61b9bb5c458
> This one covers cases where the pointer is pointing to a const string,
> so really there's no sense in injecting the "%s", but I was collecting
> them to make real ones stand out.
I don't agree. Yes, a human can verify that _currently_, only "pencrypt"
and "pdecrypt" can ever reach pcrypt_sysfs_add(). But without the
compiler being smart enough to do that, one will never know if some new
caller shows up, or one of those literals grows a % for some reason.
Adding "%s" doesn't cost much, especially not in cases (like this one)
where the fmt+args end up at kobject_set_name_vargs - for a "%s" +
literal that does a (succesful) kstrdup_const(), so we never even hit
the vsnprintf() engine.
>> Patches 5,6,7 are
>> some examples of where one might add fmtcheck() calls. I don't think
>> we can get to a state where we can unconditionally add
>> -Wformat-nonliteral to the build, but I think there's a lot of
>> low-hanging fruit.
>
> How much work do you think it'd take to get to a
> format-nonliteral-clean build? I think it's worth doing the work if
> it's not totally intractable.
Probably less than the VLA removal. But it kind of depends on which
tools one allows. I can't see how to do it without something like
fmtcheck() to annotate certain places (e.g. the nfs example). Maybe a
no_fmtcheck() to annotate places which have been manually verified
[modulo the above "but that may change..."] would also be needed
(no_fmtcheck would be the same as fmtcheck for at !CONFIG_FMTCHECK
kernel), similar to how we have no_printk.
I kind of agree with Guenther that the hwmon example is a bad one. It
would be better to have the compiler check all those string literals
against a pattern at build time. Probably the format template plugin can
be extended to apply to any "const char*" declaration, not just those
sitting inside structs. But I'd rather get fmtcheck() in first before
returning to work on that plugin.
Rasmus
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